Immune system homeostasis relies on a fine balance between a variety of T cell populations, including effector CD8 and CD4 T cells and regulatory T cells. In disease states however, such as cancer and autoimmune disease, this balance can be perturbed. In cancer, infiltrating anti-tumor cytotoxic T cells can be prevented from attacking cancer cells by tumor-resident regulatory T cells. This can be seen from analysis of most human tumors, in which there is a significant correlation between immune infiltration by cytotoxic T cells and improved outcome, whereas infiltration by regulatory T cells is instead associated with a poor outcome. Indeed, several studies have demonstrated prognostic significance of the CD8/Treg tumor ratio. Numerous mouse models have shown that depletion of Treg with anti-CD25 antibody before tumor implantation can have a dramatic impact on prevention of tumor growth. In autoimmune diseases, effector T cells remain unregulated and attack the body's own tissues. A major premise in this regard is that defects in Treg cell number or function are a contributing factor. Therefore, the ability to alter the balance between cytotoxicity and regulation by fine-tuning the T cell response has great potential for the treatment of cancer, autoimmune, and other diseases.
One approach to controlling the balance of effector to regulatory T cells is to target the Treg population for direct modulation. However, despite years of effort, the discovery of a single Treg-specific surface marker has been elusive, frustrating efforts to deplete them specifically with monoclonal antibodies.
Effector versus regulatory T cells can be loosely identified by their surface markers, which can change based on their activation state. Cytotoxic T cells express CD8, which interacts with class I MHC. Effector helper T cells express CD4, which interacts with class II MHC on antigen-presenting cells. The hallmark of Treg cells is constitutive expression of both CD4 and CD25. CD25 is the alpha component of the IL2 receptor (IL2Rα), which, together with CD122 (IL2Rβ) and the common cytokine receptor γ-chain (yc) (CD132) form the trimeric high-affinity receptor for IL2. Several approaches have attempted Treg-specific depletion by targeting the high-affinity IL2 receptor CD25 (IL2Rα) with anti-CD25 antibodies such as daclizumab, or with IL2-diptheria toxin (IL2-DT) fusions. However, CD25 alone is not an ideal target because it also expressed on CD8 and CD4 effector T cells upon activation. Thus, approaches that target Treg CD25 by itself might defeat their own purpose by also depleting the activated effector cells that could potentially attack the tumor.
Because of the importance of IL2 for T cell proliferation and homeostasis, a variety of approaches to T cell modulation have utilized IL2 itself or blocking of its high-affinity receptor component CD25. Systemic IL2 administration (Proleukin) is an approved therapy for metastatic melanoma and metastatic renal cell carcinoma based on its ability to promote expansion of effector T cells. However, systemic IL2 administration is also expected to promote the suppressive Treg population, potentially diminishing or confounding the desired enhancement of cytotoxic T cells. Furthermore, systemic IL2 administration is also associated with a variety of toxicities. Patients receiving systemic IL2 treatment frequently experience severe cardiovascular, pulmonary, renal, hepatic, gastrointestinal, neurological, cutaneous, haematological and systemic adverse events. The majority of these side effects can be explained by the development of so-called vascular leak syndrome (VLS), a pathological increase in vascular permeability leading to pulmonary edema and other issues. There is no treatment of VLS other than withdrawal of IL2. These problems have led to the pursuit of IL2 variants that perturb its affinity for one or more of its receptor subunits. Alternatively, anti-CD25 antibodies that block IL2-mediated T cell expansion have been utilized to treat various diseases. Zenapax (daclizumab) is an approved immunosuppressant for organ transplantation and is being investigated for the treatment of autoimmune diseases such as multiple sclerosis. These uses were developed based on daclizumab's presumed ability to reduce effector T cell responses. However, due to the more recently recognized dependence of Treg on IL2 for survival, daclizumab is now—somewhat paradoxically—being utilized in efforts to reduce Treg numbers in oncology. Because of the strong potential of either IL2 or anti-CD25 agents to promote or reduce both effector T cells and Treg with limited selectivity, there is a strong need in the field to create more selective Treg modulators.